'In a nutshell' means a very short succinct precis or summary of something. So 'my life in a nutshell' would be 2 or 3 sentences summarising your life (so far!)

my life is in a nutshell,that's mean that u r can't do anything,u need serious help and u r being cornered that's what i know from those "nutshell"

It's “ 'my' life in a nutshell” and the idiom of in a nutshell can be used for anything! It means without long explanations, in sum.